The term oxymethylene polymer as used herein is meant to include oxymethylene homopolymers and diethers and diesters. Also included are oxymethylene copolymers, which include oxymethylene polymers having at least 60 percent recurring oxymethylene units and at least one other unit derived from a monomer copolymerizable with the source of the oxymethylene units.
Oxymethylene polymers having recurring --CH.sub.2 O-- units have been known for many years. They may be prepared for example, by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde, and will vary in physical properties such as thermal stability, molecular weight, molding characteristics, color and the like depending, in part, upon their method of preparation, on the catalytic polymerization technique employed and upon the various types of comonomers which may be incorporated into the polymer.
While the high molecular weight oxymethylene polymers are relatively thermally stable, various treatments have been proposed to increase the polymers utility by increasing thermal stability. Among these are end capping of hemiformal groups of polyoxymethylene homopolymers and hydrolysis to remove unstable groups of oxymethylene in copolymers containing interspersed stable units, such as ethoxy groups. Even beyond these treatments, it has been found necessary to incorporate various stabilizers, antioxidants and chain-scission inhibitors into the polymers.
Unfortunately oxymethylene polymers are subject to degradation, particularly under the influence of heat. The degradation results mainly from the following three processes:
1. Thermal degradation of the chain end with liberation of gaseous formaldehyde. This degradation which takes place largely under the influence of heat, is often obviated by the presence of either an ether or an ester group at the end of the polymer chain.
2. Oxidative attack leading to chain scission and depolymerization. This is often retarded by the addition of antioxidants to the composition such as compounds containing phenolic or amino groups.
3. Acidolytic cleavage of the chain may occur which also liberates formaldehyde. Acidolytic degradation arises from the presence of acidic species originating from one of several sources: (A) acidic catalyst residues which may have been used in preparation of the polymer, (B) formic acid formed in situ when the trace quantities of formaldehyde generated in processing are oxidized, and (C) acetic acid generated from acetate end groups when a given chain, so stabilized, depolymerizes as a result of occasional oxidative or acidolytic chain scission. To alleviate this condition and to prevent degradation of the polyoxymethylene copolymer especially during subsequent processing in the hot state, "formaldehyde acceptors" or "acid scavengers" are often admixed with the polymer composition. Among the compounds which can be used for this purpose are hydrazines and their derivatives, ureas, certain amides and diamides, polyamides, and metallic salts of acetic acid and fatty acids.
Among the most successful and widely used thermal stabilizers are nitrogen-containing compounds which function as formaldehyde acceptors and acid scavengers. These have been found to be effective in lowering the thermal degradation rate of the polymer.
A particularly preferred stabilizer and one that has found use in commercial applications is characterized as a superpolyamide. The superpolyamide stabilizers are the macromolecular superpolyamides, commonly known as nylons, that have carboxamide linkages ##STR1## as integral portions of their polymer chains. These superpolyamides preferably have melting points below approximately 220.degree. C., in which R represents a hydrogen atom, a lower alkyl group, or a lower alkoxy group, and have a degree of polymerization of approximately 100 to 200. A preferred group of the superpolyamides includes those condensation polymers that on hydrolysis yield either omega-aminocarboxylic acids or mixtures of dicarboxylic acids and diamines.
It has been taught that the various nitrogen-containing stabilizers, e.g., polyamides, poly(vinylpyrrolidone), the various acrylamide copolymers, melamine, cyanoguanamine, nitrilotrispropionamide and the like that have been added as acid scavengers have been found to cause an amine-like or fishy odor, which is undesirable when using the oxymethylene polymer as a container for packaging consumer goods. Thus, various metal salts of non-nitrogenous organic carboxylic acids and alcohols have been suggested as a thermal or chain-scission inhibitor for use with oxymethylene polymers in place of the nitrogen-containing stabilizers. For example, U.S. Pat. No. 3,488,303 discloses using a salt selected from the group consisting of lanthanide metal salts of non-nitrogenous organic carboxylic acids and alcohols, while U.S. Pat. No. 3,484,399 and '400 disclose using metal salts of non-nitrogenous organic acids and alcohols in which the salt is prepared from alkali or alkaline earth metals, zinc, aluminum, tin and other metals. It has been found, however, that at the levels required to prevent substantial thermal degradation of the oxymethylene polymer, the metal salts greatly discolor the molded articles.
Accordingly, there is a need to improve the thermal stabilization of oxymethylene polymers and, in particular, to improve upon the thermal degradation properties of an oxymethylene polymer which has been stabilized with a superpolyamide. The present invention is directed to such need.